linux-kernel - Re: [PATCH 1/1] powerpc/pseries/iommu: Fix window size for direct mapping with pmem

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Message-ID: <58b28a98-37aa-055f-5dec-d8c0005c9519@ozlabs.ru>
Date:   Tue, 20 Apr 2021 15:18:36 +1000
From:   Alexey Kardashevskiy <aik@...abs.ru>
To:     Leonardo Bras <leobras.c@...il.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] powerpc/pseries/iommu: Fix window size for direct
 mapping with pmem



On 20/04/2021 14:54, Leonardo Bras wrote:
> As of today, if the DDW is big enough to fit (1 << MAX_PHYSMEM_BITS) it's
> possible to use direct DMA mapping even with pmem region.
> 
> But, if that happens, the window size (len) is set to
> (MAX_PHYSMEM_BITS - page_shift) instead of MAX_PHYSMEM_BITS, causing a
> pagesize times smaller DDW to be created, being insufficient for correct
> usage.
> 
> Fix this so the correct window size is used in this case.

Good find indeed.

afaict this does not create a huge problem though as 
query.largest_available_block is always smaller than (MAX_PHYSMEM_BITS - 
page_shift) where it matters (phyp).


Reviewed-by: Alexey Kardashevskiy <aik@...abs.ru>


> Fixes: bf6e2d562bbc4("powerpc/dma: Fallback to dma_ops when persistent memory present")
> Signed-off-by: Leonardo Bras <leobras.c@...il.com>
> ---
>   arch/powerpc/platforms/pseries/iommu.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/platforms/pseries/iommu.c b/arch/powerpc/platforms/pseries/iommu.c
> index 9fc5217f0c8e..836cbbe0ecc5 100644
> --- a/arch/powerpc/platforms/pseries/iommu.c
> +++ b/arch/powerpc/platforms/pseries/iommu.c
> @@ -1229,7 +1229,7 @@ static u64 enable_ddw(struct pci_dev *dev, struct device_node *pdn)
>   	if (pmem_present) {
>   		if (query.largest_available_block >=
>   		    (1ULL << (MAX_PHYSMEM_BITS - page_shift)))
> -			len = MAX_PHYSMEM_BITS - page_shift;
> +			len = MAX_PHYSMEM_BITS;
>   		else
>   			dev_info(&dev->dev, "Skipping ibm,pmemory");
>   	}
> 

-- 
Alexey